The present invention relates to improvements of an exposure time control circuit for an electric shutter for a camera using an internal photometric type digital memory reproducing system.
The exposure time control circuit for an electric shutter a camera proposed under the application No. Sho-48-95007 by the applicants is constructed so that a counting circuit is reset by the reset pulse generated from a reset pulse generating circuit composed of a mono-stable multi-vibrator connected to operate in synchronizing with a power supply after actuation of the mechanical power switch cooperative with the release button in operation, thereby allowing standard pulses to start generating from the standard pulse generating circuit by the trailing edge of this reset pulse and also actuating the brightness/time conversion circuit which generates a single pulse having a pulse duration corresponding with the brightness of a subject. The standard pulses being generated in the standard pulse generating circuit concurrently with the generation of the single pulse are delivered as inputs to the counting circuit for counting, and concurrent with the completion of the single pulse the inputs to the counting circuit are stopped thus stopping memory operation, and the system is then transfered electrically from the state capable of memorizing to that capable of calling or reading-out by the trailing edge of this single pulse. By depressing the release button more, the mirror for a view finder goes up starting exposure and simultaneously allows a trigger pulse to be generated by actuation of the timing switch interlocked with the mirror's up-going motion, and with generation of this trigger pulse, a calling of the memory value so far memorized in the counting circuit is started and a shutter-closing signal is then generated completing exposure at the end of calling.
In the system as described above, if the release button is pushed down to the full extent before completion of the single pulse in case the pulse width of single pulses generated in the brightness/time conversion circuit is long, for example, when its brightness of a subject to be photographed is low, etc., it is probable that the mirror will go up starting exposure. Since the light passed through a photographing lens will then no longer fall upon the light-receiving element, the single pulse will not come to an end in which case the system will fail to be ready for being read-out or called up. Instead, memory operation will continue and the shutter will remain open without completing exposure.